....Perhaps the most well-known human study was done by Mayer, the UCLA researcher. He recruited 25 subjects, all healthy women; for four weeks, 12 of them ate a cup of commercially available yogurt twice a day, while the rest didn’t. Yogurt is a probiotic, meaning it contains live bacteria, in this case strains of four species, bifidobacterium, streptococcus, lactococcus, and lactobacillus. Before and after the study, subjects were given brain scans to gauge their response to a series of images of facial expressions—happiness, sadness, anger, and so on.
“This was not what we expected, that eating yogurt twice a day for a few weeks would do something to your brain.”
To Mayer’s surprise, the results, which were published in 2013 in the journal Gastroenterology, showed significant differences between the two groups; the yogurt eaters reacted more calmly to the images than the control group. “The contrast was clear,” says Mayer. “This was not what we expected, that eating a yogurt twice a day for a few weeks would do something to your brain.” He thinks the bacteria in the yogurt changed the makeup of the subjects’ gut microbes, and that this led to the production of compounds that modified brain chemistry.
It’s not yet clear how the microbiome alters the brain. Most researchers agree that microbes probably influence the brain via multiple mechanisms. Scientists have found that gut bacteria produce neurotransmitters such as serotonin, dopamine and GABA, all of which play a key role in mood (many antidepressants increase levels of these same compounds). Certain organisms also affect how people metabolize these compounds, effectively regulating the amount that circulates in the blood and brain. Gut bacteria may also generate other neuroactive chemicals, including one called butyrate, that have been linked to reduced anxiety and depression. Cryan and others have also shown that some microbes can activate the vagus nerve, the main line of communication between the gut and the brain. In addition, the microbiome is intertwined with the immune system, which itself influences mood and behavior.
This interconnection of bugs and brain seems credible, too, from an evolutionary perspective. After all, bacteria have lived inside humans for millions of years. Cryan suggests that over time, at least a few microbes have developed ways to shape their hosts’ behavior for their own ends. Modifying mood is a plausible microbial survival strategy, he argues that “happy people tend to be more social. And the more social we are, the more chances the microbes have to exchange and spread.”